The present invention relates to a non-ablative method and apparatus for drilling stopped or through orifices beginning at any depth, or in any one of a set of stacked wafers, plates or substrates, primarily, but not limited to, such transparent materials as glass, sapphire, silicon such that the structural characteristics of the orifice and surrounding material exceed that found in the prior art.
There is a huge demand for drilling multiple holes in a transparent substrate such as one made of glass or a polycarbonate. One application of a drilled substrate is for use as a filter for air monitoring, particle monitoring, cytology, chemotaxis, bioassays, and the like. These commonly require orifices a few hundred nanometers to tens of micrometers in diameter that remain identical to each other and have a hole to surface area ratio that remains stable when produced in volume.
Currently, the prior art material processing systems produce orifices in substrates such as glass by diamond drilling, or laser exposure techniques such as: ablative machining; combined laser heating and cooling; and high speed laser scribing. All of the prior art systems have low throughput times, do not work well with many of the new exotic substrate materials, have problems with the opacity of multiple level substrate stacks, cannot attain the close orifice spacing sought, propagate cracks in the material or leave an unacceptable surface roughness on the orifice sides and surface surrounding the point of initiation as detailed below.
In current manufacturing, the singulation, treatment of wafers or glass panels to develop orifices typically relies on diamond cutting routing or drilling.
Laser ablative machining is an active development area for singulation, dicing, scribing, cleaving, cutting, and facet treatment, but has disadvantages, particularly in transparent materials, such as slow processing speed, generation of cracks, contamination by ablation debris, and moderated sized kerf width. Further, thermal transport during the laser interaction can lead to large regions of collateral thermal damage (i.e. heat affected zone). While laser ablation processes can be dramatically improved by selecting lasers with wavelengths that are strongly absorbed by the medium (for example, deep UV excimer lasers or far-infrared CO2 laser), the above disadvantages cannot be eliminated due to the aggressive interactions inherent in this physical ablation process.
Alternatively, laser ablation can also be improved at the surface of transparent media by reducing the duration of the laser pulse. This is especially advantageous for lasers that are transparent inside the processing medium. When focused onto or inside transparent materials, the high laser intensity induces nonlinear absorption effects to provide a dynamic opacity that can be controlled to accurately deposit appropriate laser energy into a small volume of the material as defined by the focal volume. The short duration of the pulse offers several further advantages over longer duration laser pulses such as eliminating plasma reflections and reducing collateral damage through the small component of thermal diffusion and other heat transport effects during the much shorter time scale of such laser pulses. Femtosecond and picosecond laser ablation therefore offer significant benefits in machining of both opaque and transparent materials. However, machining of transparent materials with pulses even as short as tens to hundreds of femtosecond is also associated with the formation of rough surfaces and microcracks in the vicinity of laser-formed orifices or trench that is especially problematic for brittle materials like Alumina glasses, doped dielectrics and optical crystals. Further, ablation debris will contaminate the nearby sample and surrounding surfaces.
A kerf-free method of cutting or scribing glass and related materials for orifices relies on a combination of laser heating and cooling, for example, with a CO2 laser and a water jet. Under appropriate conditions of heating and cooling in close proximity, high tensile stresses are generated that induces cracks deep into the material, that can be propagated in flexible curvilinear paths by simply scanning the laser cooling sources across the surface. In this way, thermal-stress induced scribing provides a clean separation of the material without the disadvantages of a mechanical scribe or diamond saw, and with no component of laser ablation to generate debris. However, the method relies on stress-induced crack formation to direct the scribe and a mechanical or laser means to initiate the crack formation. Short duration laser pulses generally offer the benefit of being able to propagate efficiently inside transparent materials, and locally induce modification inside the bulk by nonlinear absorption processes at the focal position of a lens. However, the propagation of ultrafast laser pulses (>5 MW peak power) in transparent optical media is complicated by the strong reshaping of the spatial and temporal profile of the laser pulse through a combined action of linear and nonlinear effects such as group-velocity dispersion (GVD), linear diffraction, self-phase modulation (SPM), self-focusing, multiphoton/tunnel ionization (MPI/TI) of electrons from the valence band to the conduction band, plasma defocusing, and self-steepening. These effects play out to varying degrees that depend on the laser parameters, material nonlinear properties, and the focusing condition into the material.
There are other high speed scribing techniques for flat panel display (FPD) glasses. A 100-kHz Ti:sapphire chirped-pulse-amplified laser of frequency-doubled 780 nm, 300 fs, 100 μJ output was focused into the vicinity of the rear surface of a glass substrate to exceed the glass damage threshold, and generate voids by optical breakdown of the material. The voids reach the back surface due to the high repetition rate of the laser. The connected voids produce internal stresses and damage as well as surface ablation that facilitate dicing by mechanical stress or thermal shock in a direction along the laser scribe line. While this method potentially offers fast scribe speeds of 300 mm/s, there exists a finite kerf width, surface damage, facet roughness, and ablation debris as the internally formed voids reach the surface.
Although laser processing has been successful in overcoming many of the limitations associated with diamond cutting, as mentioned above, new material compositions have rendered the wafers and panels incapable of being laser scribed.
Henceforth, a fast, economical system for drilling through or stopped orifices in transparent materials emanating from the top or bottom surface, that avoids the drawbacks of existing prior art systems would fulfill a long felt need in the materials processing industry. This new invention utilizes and combines known and new technologies in a unique and novel configuration to overcome the aforementioned problems and accomplish this.